This invention relates to a supercharging control method for internal combustion engines, which is directed to improvement of the rising characteristic of supercharging pressure in the engine.
There have conventionally been proposed methods of controlling supercharging pressure in an internal combustion engine equipped with a supercharger capable of controlling the supercharging pressure such as a variable capacity turbocharger utilizing an exhaust gas flow for driving a turbine thereof, by means of an actuator such as a pressure-responsive type actuator including a diaphragm which operates in response to supercharging pressure or vacuum within the intake pipe of the engine, or an actuator utilizing a stepping motor. Amongst these methods, a method has been proposed by the assignee of the present application, e.g. by Japanese Provisional Patent Publication (Kokai) No. 63-129126, which controls the supercharging pressure in open loop control mode when the supercharging pressure is in a transient state where it drastically changes, and in feedback control mode when it is in a steady state. This proposed method is capable of controlling the supercharging pressure more stably than conventional methods by virtue of the open loop control in the transient state. However, the proposed method still has room for further improvement in respect of the accelerability of the engine.
Specifically, according to the proposed method, the open loop control is executed at acceleration of the engine, which is a transient state and in which the supercharging pressure should be promptly increased. During the open loop control, a basic control amount by which the supercharging pressure is controlled is determined in accordance with operating conditions of the engine, e.g. as a function of throttIe valve opening and engine rotational speed, by the use of a map of these parameters, or like means, and the determined basic control amount is directly applied to the open loop control. However, according to this control manner it is difficult to properly control the supercharging pressure in the case where it is increasing at a progressively increasing rate.
Generally, when the accelerator pedal of a vehicle is stepped on for acceleration of the vehicle, the supercharging pressure does not rise at a constant rate, but it rises initially at a low rate and then at a progressively increasing rate, sometimes resulting in an excessive rise rate. This phenomenon is more conspicuous at sudden standing start of the vehicle or at rapid acceleration where usually the supercharging pressure is controlled to increase to a high value at a high rate so as to enhance the control responsiveness. However, such an excessive rise rate of supercharging pressure at acceleration can cause a sudden increase in the engine torque, which spoils the feeling of smooth acceleration and can also cause spinning of wheels of the vehicle. Further, in transition to the feedback control mode the excessive rise rate of supercharging pressure can cause overboosting, which in turn can cause knocking of the engine, also spoiling the driveabilitY at acceleration.
On the other hand, if the rise rate of supercharging pressure is too low at acceleration, this gives the driver a feeling of insufficient acceleration, failing to obtain a feeling of smooth acceleration.